The field of art to which this invention pertains is the removal of high molecular weight reaction by-products from a selective hydrogenation reaction zone effluent.
Processes for the production, recovery and purification of butadiene are known and butadiene is useful for the production of synthetic rubber. Crude butadiene streams are produced from the thermal and catalytic cracking of hydrocarbons. The production of crude butadiene streams also produces undesirable co-products such as ethyl acetylene and vinyl acetylene, for example. The explosive nature of the acetylenes and their tendency to form and deposit polymers calls for special precautions when handling the hydrocarbon stream containing acetylene. In addition, when a butadiene-rich stream is used for the production of synthetic rubber, the presence of acetylene is highly undesirable and must be removed. In order to minimize and reduce the difficulties presented by the presence of acetylenes, the crude butadiene streams are selectively hydrogenated in a selective hydrogenation zone operated to convert the acetylene to diolefins. However, the selective hydrogenation of acetylene produces trace quantities of heavy reaction by-products commonly referred to as polymers or xe2x80x9cgreen oilxe2x80x9d. In a stand alone selective hydrogenation process for the hydrogenation of acetylenes, the co-produced green oil was removed from the desired butadiene steam by means of a fractionation column having all of the conventional components including a reboiler, overhead condenser and receiver, and a reflux pump.
U.S. Pat. No. 4,049,742 (Weitz et al) discloses a process wherein 1,3-butadiene is recovered with the aid of a selective solvent from a C4 hydrocarbon mixture containing 1,3-butadiene, hydrocarbons which are more soluble in the selective solvent than 1,3-butadiene, including acetylenes and possibly 1,2-butadiene and C5 hydrocarbons, and hydrocarbons which are less soluble in the selective solvent than 1,3-butadiene. The crude C4 hydrocarbon mixture is separated by the use of one or more extractive distilling zones into a distillate containing the less soluble hydrocarbons, a stream consisting of 1,3-butadiene and a stream containing the more soluble hydrocarbons including the higher acetylenes and 1,3-butadiene.
In accordance with the present invention, it has been discovered that when the liquid effluent from a selective hydrogenation reaction zone containing crude butadiene and trace amounts of heavy reaction by-products is vaporized in preparation for subsequent extraction of butadiene in a selective solvent process, a stream rich in heavy reaction by-products is isolated and recovered. The present invention utilizes a vaporizer containing a fractionation zone which is refluxed with a liquid raffinate process stream from the solvent extraction zone.
It has now been found that an improved process for the rejection of heavy reaction by-products from a selective hydrogenation reaction zone effluent can be achieved by introducing the selective hydrogenation effluent into a butadiene extraction vaporizer containing a fractionation zone which is refluxed with a raffinate stream from the butadiene extraction zone. This produces a vaporized stream containing butadiene and having a reduced concentration of heavy reaction by-products, and a concentrated liquid product stream containing heavy reaction by-products.
One embodiment of the present invention is a process for the rejection of heavy reaction by-products from a selective hydrogenation reaction zone effluent containing butadiene and trace amounts of heavy reaction by-products which process comprises: (a) introducing the selective hydrogenation reaction zone effluent into a butadiene extraction vaporizer containing a fractionation zone; (b) refluxing the fractionation zone with a raffinate stream from a butadiene extraction zone; (c) removing a vaporized stream containing butadiene and having a reduced concentration of heavy reaction by-products from the vaporizer; (d) removing and recovering a concentrated liquid product stream containing heavy reaction by-products from the vaporizer; and (e) introducing the vaporized stream containing butadiene into the butadiene extraction zone.
Other embodiments of the present invention encompass further details such as fractionation zone details and operating conditions.